Technical Field
This disclosure relates to voltage regulators and more particularly to a method of preventing inversion of output current flow in a voltage regulator and a related voltage regulator.
Description of the Related Art
Linear voltage regulators are widely used devices in modern electronic systems to provide a regulated voltage under different operating conditions for compensating variations of load current (ILOAD), functioning temperature (TA) and input voltage (VIN) provided by an unregulated power supply.
A basic scheme of a LDO (Low Drop-Out) voltage regulator is shown in FIG. 1. It comprises a power transistor 201 in a voltage follower configuration, inserted in an output electric path from a supply line at an unregulated input voltage VIN and an output terminal on which an output regulated voltage VOUT is made available. An error operational amplifier 200 controls the power transistor 201 with an error voltage corresponding to the difference between a reference voltage VREF, representing a nominal value of the output voltage VOUT, and a scaled replica of the voltage VOUT available on the center tap of a resistive voltage divider 204, 205 connected between the output terminal and a ground terminal GND. In order to obtain low dropout performances, the error amplifier 200 is supplied by charge pump 206 to ensure a sufficiently great control voltage for power transistor 201.
The power transistor 201, that is an N-channel MOSFET in the shown example, has two intrinsic diodes 202 and 203 between the body B, drain D and source S. One of these diodes may constitute a conduction path for reverse flow of output current, if the situation is not handled. In the shown circuit, the reverse flow of the output current is handled by connecting the bulk B to a node GND at the lowest potential (ground) available in the regulator, thus keeping diodes 202, 203 always reverse biased.
With this approach, the problem of reverse current flow is only partially solved, because reverse current flow through the channel of the power MOSFET 201 is still possible. This may occur for example when the output voltage VOUT goes below its nominal level and the unregulated input voltage VIN falls below the output voltage VOUT. In this condition, the regulation loop acts to increase the output voltage VOUT up to its nominal level by forcing the gate-source voltage of the power MOSFET 201 at a maximum value, thus minimizing the on-resistance of the MOSFET channel. Therefore, the power MOSFET operates in its triode functioning region, the N-channel behaves like a resistor and the current between source S and drain D can flow in both directions depending on the polarity of the difference VOUT-VIN. If the power MOSFET is large, the on-resistance is low and thus very high reverse currents of several amperes may flow throughout the N-channel, leading to unpredictable and even destructive effects.
Moreover, the body B of the power transistor connected to the lowest potential causes body effects that influence negatively the performances of the voltage regulator.
Similar situations take place in the prior LDO regulator of FIG. 2. This LDO voltage regulator is based on P-channel power MOSFET 301 working in a common source configuration. The power transistor has two intrinsic body diodes 302, 303. During normal functioning conditions, the switch 305 is closed, thus the transistor body B is shorted to the source S, and the switches 304, 306 are open. The intrinsic diode 303 constitutes a conduction path for reverse current flow when the output voltage VOUT increases sufficiently above the input voltage VIN. To prevent this from occurring, the regulator comprises a hysteresis comparator 309 with built in offset 310, that controls the switches 304, 305, 306 as shown in the figure and enables/disables also the output stage 311 of the error amplifier 300. The hysteresis comparator 309 senses the difference between the voltages VIN and VOUT and disables the output stage 311 of the error amplifier 300 when VOUT>VIN+VOFFSET1.
The offset voltage VOFFSET1 is set to an appropriate level for ensuring that the error amplifier is not disabled during normal operation (VIN>VOUT) and that no significant current flows throughout the forward biased intrinsic diode 303 even in a design worst case condition (typically, at high temperatures).
Also in the prior regulator of FIG. 2 the problem of inversion of current flow is solved only partially, because a reverse current path may be constituted by the channel of the power MOSFET 301 when the output voltage VOUT is smaller than its nominal value and is greater than the input voltage (VOUT>VIN). In this condition, the error amplifier 300 will force the power transistor 301 in a deep conduction functioning condition, thus even a very small positive difference VOUT−VIN will cause a relatively great current to flow throughout the channel of the power MOSFET 301.